Senator Tom Coburn is trying to challenge the proposition that all tax breaks are created equal.

The effort by the Oklahoma Republican to end a tax credit for ethanol places him in conflict with farm-state senators who want to keep the tax incentive. It also puts him at odds with tax-cut advocates in the Republican Party, who argue that eliminating the tax break would result in an unacceptable tax increase. The proposal could reach the Senate floor this week.

A vote would force many Republicans to choose between their no-tax-increase pledge and their desire to reduce the budget deficit by cutting what they view as waste. Coburn's amendment reaches beyond the ethanol debate and draws attention to tax expenditures, which reduce federal revenue by more than $1 trillion a...